1. Field of the Invention
This invention relates to a system for controlling the charging and purging cycles of a compressor and an air dryer used in a compressed air system.
2. Description of prior Art
Compressed air systems are commonly used to operate the brakes of heavy duty vehicles, and operate other vehicle accessories (such as fan clutches). Compressed air systems also may be used with industrial machines. Compressed air systems commonly include an air dryer containing a desiccant material which removes the moisture contained in the compressed air before the compressed air is stored in a reservoir. The desiccant used in air dryers must be periodically purged of its moisture by backflowing a quantity of compressed air through the desiccant and discharging the air to atmosphere. Prior art air dryers commonly included an integral purge volume within the air dryer housing in which a quantity of compressed air was kept segregated for use in purging. Of course, the integral purge volume substantially increases the size of the air dryer, thereby making installation on some vehicles difficult. Alternatively, some air dryers use a remote mounted purge volume, in which a canister separate from the system reservoirs is mounted remote from the air dryer and is connected thereto by a separate air line. A remote purge volume substantially increases system cost, because of the cost of the added component, and also because the remote purge volume makes installation of the air dryer on a vehicle more complex and time consuming.
In general, the air stored in the reservoir is not used for purging, because the purge control valve on air dryers is open to atmosphere for a substantial time period, which would cause the pressure level in the reservoir to be depleted to an unacceptable level. Although it has been proposed that pneumatic and/or electrical timers be used to limit the amount of air taken from a storage reservoir during purging, each of these proposed devices has practical difficulties. Limiting the length of time that air is taken from a reservoir is generally not a satisfactory solution to the problem because the amount of compressed air passing through the system during a purge cycle of a given period of time varies depending on several factors, including the reservoir volume, the pressure level to which the reservoir was initially charged (which may vary substantially), and the size of the orifice through which the purge air is communicated when the desiccant is purged.